The invention relates to a pressure standard device as set forth in the classifying portion of the claim.
The book "Messen, Steuern und Regeln in der chemischen Technik" , edited by J. Hengstenberg, B. Sturm, O. Winkler, third edition, volume I, pages 112 to 117, Berlin 1980, discloses piston manometers in which a cylindrical piston engages through an opening into a closed oil-filled cylinder to which the pressure p is applied by way of a further opening. The vertically directed piston is loaded with weights. Problems arise in that arrangement with regard to sealing of the piston in the opening so that no friction may occur and at the same time no fluid can to escape. The piston is therefore usually caused to rotate prior to the pressure being applied, and that reduces the level of friction. The friction between the piston and the cylinder can be further reduced by air or fluid being radially introduced under pressure into the clearance. A further problem arises when measuring pressure in gases when making the transition from the fluid to the gas, in which respect special precautions must be taken if the gas involved is not an inert gas. Even then making the transition from the fluid to the gas gives rise to problems as the position in respect to the height of the piston in the cylinder is not defined. It is therefore necessary to provide stops with relation to excessive movement of the piston both in an upward direction and in a downward direction.
German laid-open application (DE-OS) No. 35 14 911 describes another pressure measuring device for measuring a pressure applied to a piston by way of a gaseous medium, wherein the piston is guided in a cylinder in a practically friction-free manner in bearings by virtue of the fact that the bearings are supplied with a fluid under pressure, the piston acting on a force-measuring means. The bearing fluid which escapes into the cylinder chamber containing the gas is removed in a regulated manner in that arrangement. In an alternative embodiment, a gas under pressure is supplied to the bearings, while bearing gas which issues into the cylinder chamber can be used for building up the pressure in a conduit which is connected to the cylinder chamber. As the force-measuring means operates practically without performing any travel movement, practically no movement of the piston also occurs and the position of the piston plays no part.
The GB-A-2056098 discloses a standardized pressure instrument comprising a cylinder and a piston guided therein the height position thereof having to be maintained constant irrespective of a varying loading of the piston in order to prevent a contact to the cylinder. In the lower region of the cylinder a pressure space is formed being connected to a pressurized fluid via a duct and by loading of the piston with a calibration mass the pressure in the pressure space may be determined. A constant height of zero position, respectively, of the piston is determined by an indicating device and is maintained by supplying fluid to the pressure space.
However, the guidance of the piston by means of upper and lower diaphragms having the piston tightly clamped therebetween is disadvantageous. In view of this type of support the readings are falsified due to the deformation resistance of the diaphragms and deteriorate the measuring accuracy of the force measuring device. In view of the diaphragms fixedly clamping the piston the latter may move minimal only such that the height regulation for maintaining a predetermined height position must be designed extremely accurate. Furthermore, the expenditures for these diaphragms are considerable.